Tyres were fitted a day early (yesterday). My oh my I should have done this weeks ago, much improved, I can't believe by how much. The steering feels more precise, going over bumps feels more controlled and the whole thing just feels more refined. To recap, I have changed from 275/40/20 nasty £60 per corner tyres (that still had 5mm of tread on them) to 245/45/20 premium all season tyres keeping the 8.5J wheels.
Three things have changed, 30mm narrower tyre (the sidewall height of a 275/40 is within 1mm of a 245/45 so effectively the same), move from very cheap to premium tyres and changing to the ideal width tyre for the fitted wheel. Which of these things has made the greatest difference I wonder? For the improvement in steering feel, I am convinced that is is having the correct width tyre on the wheel. The previously fitted oversize tyre forces the sidewalls outwards and distorts them; I am certain this affects the steering feel and adds a level of 'bounce' into the tyre because the sidewall is compromised, which is unpleasant when driving.
Early days and only my subjective opinion. But the more I think about it, the commonplace practice of fitting 275 or 265 width tyres onto 8.5J wide wheels may well be more of a compromise than I first thought.